Refrigerator with forced-ventilation evaporator

ABSTRACT

A refrigerator having a compressor, a condenser and an evaporator connected in a refrigerant circuit, with the evaporator being forced-ventilated by a fan with adjustable power, the refrigerator including a control unit configured to regulate the power of the fan in inverse proportion to a thermal load of the refrigerator.

The present invention relates to a refrigerator with a compressor, a condenser and an evaporator which are connected in a refrigerant circuit, with the evaporator being forced-ventilated by a fan of adjustable power. Such a refrigerator is known from DE 101 39 834 A1.

With this known refrigerator two evaporators, which cool two compartments of a refrigerator, are connected in series in the refrigerant circuit, with a fan being assigned to one of the two evaporators. The selective operation or non operation of the fan while the compressor is running creates the opportunity of modulating the distribution of air to the two compartments. This takes account of the fact that the ratio of the demand from the compartments held at various temperatures to cooling power differs as a function of ambient temperature. If the appliance is operated in a warm environment, both compartments demand significant cooling power; In a cold environment the demand for cooling power of the warmer of the two compartments is proportionately significantly. lower. The high demand for cooling power of the warmer compartment in a warm environment can be satisfied by its evaporator being forced-ventilated by the fan.

When a refrigeration device is operating in stationary mode, its evaporator is at a lower temperature than the ambient temperature, so that a considerable proportion of the coolant circulating in the coolant circuit can be stored in the evaporator at low pressure. This storage effect also restricts the pressure of the refrigerant in the warm parts of the coolant circuit, against which the compressor is working and thereby the power output by the compressor. If however a refrigerator is being put onto operation and its evaporator as at ambient temperature, the pressure in the coolant circuit is significantly higher than in stationary operation and the compressor power needed to achieve the stationary operating state is also correspondingly high. If the compressor is not designed to be able to deliver this power it can occur that the evaporator is not successfully cooled down. A condenser which is too narrowly dimensioned can also lead to problems in such a situation if its heat-exchanging power is not sufficient to sufficiently cool down the refrigerant pushed into it and very greatly heated up before it enters into the evaporator. To exclude this danger, it is conventionally necessary to dimension compressor and condenser generously enough to enable them to guarantee that the refrigeration device is started up even under extreme conditions. Such generous dimensioning is not necessary for the operation of the refrigerator under stationary conditions, but demands space and makes for high production costs.

The object of the invention is to create a refrigerator which guarantees a reliable startup and in doing so allows compact dimensioning of compressor and condenser.

The object is achieved, for a refrigerator with a compressor, an evaporator and a condenser which are connected up in a refrigerant circuit, with the evaporator being forced-ventilated by a fan with adjustable power, by a control unit being provided which is configured to regulate the power of the fan in the opposite sense to a thermal load of the refrigerator.

In stationary operation the thermal load of the refrigeration device is far lower than when, on startup of the refrigeration compartment of the refrigerator, cooling down has to be undertaken from the ambient temperature. The invention thus makes provision for the fan to operate at higher power under stationary conditions than during startup. Because the fan is in operation under stationary conditions, it increases the surface-related heat exchanging power of the evaporator, so that the inventive refrigerator makes do with a small evaporator of which the cooling power in the unventilated state might possibly not be sufficient to keep the storage compartment at its required temperature. Its cooling power in the unventilated or only slightly ventilated state is sufficient however in each case to lower the temperature of the storage compartment to below the ambient temperature. Because the fan output is kept low when the appliance starts up, it succeeds in cooling the evaporator sufficiently so that under stationary conditions it can accept a large volume of items to be cooled and the pressure in the refrigerant circuit is reduced. Together with the pressure the power generated by the compressor is also reduced, so that this unit can be dimensioned in a compact and cost-effective manner.

Different ways can be used to estimate whether the thermal load to be managed by the refrigerant circuit is high or low. An especially simple way is to equip the control unit with timing means to record the elapsed time since the refrigeration device was switched on and to configure it in order to decide on the basis of the recorded time whether a high thermal load is present or not, with this being based on the simple premise of whether, after a prespecified time has elapsed, a significant part of the initially present heat volume has been removed from the refrigerating compartment of the appliance and the thermal load is correspondingly reduced. Such a decision is not always reliable, since situations such as a brief interruption to the mains power supply can also occur which are recorded as the refrigeration device being switched off and switched back on again, without a high thermal load necessarily having to be present as a result. This is however not any more critical since in such a case the reduction of the fan power merely leads to a temporary reduction in the cooling power of the evaporator, but within the framework of a thermostatic control of the compartment temperature in general, this is compensated for automatically by a longer run time of the compressor, without a critical increase in the compartment temperature resulting.

A decision about the presence of a high thermal load can also be made directly based on the temperature of the storage compartment. A temperature probe needed for this is conventionally present in any refrigerator. A limit value which, if exceeded, leads to the assumption that a high thermal load is present, is expediently selected between a typical setpoint temperature of the storage compartment and a typical ambient temperature. The two criteria are able to be combined with each other using logical operations. It is especially useful to only assume that there is a high thermal load if the two criteria are simultaneously fulfilled.

A temperature detected by an outside temperature sensor can be included as an alternate or further criterion for the thermal load.

Furthermore the control unit can expediently be configured to estimate a temperature of the compressor and to decide on the basis of the estimated compressor temperature whether a high thermal load is present or not. In other words the presence of a high thermal load is detected from the fact that the heating up of the compressor operating against this high thermal load is detected. A temperature sensor can be attached to the compressor for this purpose; another useful option is to estimate the compressor temperature on the basis of the ohmic resistance of an electrical conductor of the compressor, especially of a winding wire of an electric motor.

The fan power regulated by the control unit can involve an instantaneous power; preferably the regulated power is an average power of the fan, with the notification period expediently able to extend over an operating phase of the intermittently operated compressor or a part of such an operating phase. This especially enables a simple reactive power regulation by on and off sampling of the fan.

Further features and advantages of the invention emerge from the description of exemplary embodiments given below which refers to the enclosed figures. The figures are as follows:

FIG. 1 a schematic diagram of an inventive refrigerator;

FIG. 2 a flowchart of a method of operating a control unit of the refrigerator in accordance with a first embodiment;

FIG. 3 a flowchart of a method of operation in accordance with a second embodiment; and

FIG. 4 a flowchart of a method of operation in accordance with a third embodiment;

The refrigerator depicted schematically in FIG. 1 comprises a heat-insulating housing 1 with at least one refrigerating compartment and an evaporator 2 assigned to the refrigerating compartment. The evaporator 2 is connected to a compressor 3 driven by an electric motor and a condenser 4 in a refrigerant circuit. The evaporator 2 can be arranged directly adjacent to the refrigerating compartment, for example in the form of a cold-wall evaporator, with in this case a fan 5 placed freely in the refrigerating compartment being provided to drive a flow of air along the wall of the refrigerating compartment cooled by the evaporator 2; This can be a no-frost evaporator which is accommodated in a chamber displaced from the refrigerating compartment, with in this case the fan 5 being arranged in a passage between chamber and refrigerating compartment in order to promote an exchange of air between the two.

A control unit 6 is coupled to a temperature probe 7 in the refrigerating compartment and a probe 8 outside the refrigerating compartment in order, on the basis of the inside temperatures Ti detected by the temperature probes 7, 8 and the outside temperatures Ta, to control the operating state of the compressor 3 and of the fan 5.

The outside temperature probe 8 can be fitted adjacent to an outer wall of the housing 1 in order to detect a temperature obtaining in the environment of the refrigerator; it can however also be accommodated together with the compressor 3 in a machine room of the housing 1, where the temperature detected by it is influenced by the waste heat of the compressor 3.

Instead of the temperature probe 8 or to supplement this probe, the control unit 6 can also be equipped with a measurement circuit for detecting the ohmic resistance of a winding of the electric motor of the compressor 3 supplied with operating voltage via the control unit 6. Since the ohmic resistance of the winding is generally temperature-dependent, the detection of a resistance value of the winding allows a conclusion to be drawn about the temperature of the compressor 3.

A first embodiment of a method of operation executed by the control unit 6 is described with reference to FIG. 2. The method begins with the switching on of the refrigerator, by actuating a power switch, or by a supply voltage being applied to the control unit 6 in any other way (step S0). In step S1 the control unit requests the temperature Ti of the refrigerating compartment from the temperature probe 7 and compares this with a predetermined limit value Timax. Timax lies far above the temperature range to which the refrigerating compartment is set in ongoing operation and can be in the room temperature range or even above this. If the refrigeration compartment temperature Ti is less than Timax, it is assumed that the conditions for starting up the refrigeration device are normal and no protective measures must be taken to prevent an overload of the compressor 3. In this case the method jumps directly to step S7, in which a duty cycle θ=1 for the operation of the fan 5 is defined and it is operated continuously with this cycle while the compressor 3 is running.

If the inside temperature Ti lies above the limit value Timax it is to be assumed from this that a higher pressure obtains in the refrigerant circuit, operating against which over a long period the compressor 3 would be greatly stressed. In this case in step S3 a duty cycle θ<1 for the operation of the fan 5 is defined, and, while the compressor 3 is running continuously, the fan is operated intermittently with this duty cycle θ. The periods of the on and off sampling of the fan 5 can be selected to range from milliseconds to minutes. The duty cycle θ can be determined solely on the basis of the temperature Ti, with the outside temperature sensor 8 able to be dispensed with in this case. However the heat entering the device can also be taken into account with the aid of the outside temperature Ta in such a way that the smaller the value of θ selected, the higher is the outside temperature Ta.

In the event of the temperature probe 8 being subjected to the waste heat of the compressor 3, the consideration of the temperature Ta offers the additional advantage in the determination of the duty cycle θ that a disproportionate heating up caused by a high load on the compressor 3 is detected immediately and brings about the reduction of the compressor load 3 via a reduction of the duty cycle θ.

With the modified embodiment of the method shown in FIG. 3 the control unit 6 only checks in step S11, after the switch-on step S10 of the refrigerator, whether the outside temperature Ta supplied by the temperature probe 8 lies above a limit value Tamax. This limit value is expediently likewise defined in the room temperature range or above. If the temperature Ta does not lies above the limit value, normal startup conditions of the refrigerator are assumed, and in step S12 a duty cycle θ=1 is set for the fan 5 so that this operates at the same time as the compressor 3. Else a check is made in step S13 as to whether the time t elapsed since the startup step S10 is greater than a predetermined startup period t0.

If it is, this means that when the startup time has elapsed the method goes to step S12 and thereby into normal operation; else, in step S14, a duty cycle θ<1 is defined, with which the compressor 3 is then operated until the startup time interval t0 elapses. In the simplest case the duty cycle defined in step S14 can be a temperature-independent predetermined constant value; a consideration of compartment and ambient temperature Ti, Ta can be undertaken as in step S3.

The method of FIG. 3 uses step S13 to limit the maximum period of time in which, after the switching on of the appliance, it can be operated with restricted fan power, and excludes the case in which the appliance does not operate normally under unfavorable temperature conditions, in which θ=1 applies. There is however the possibility of the appliance starting up after a power failure with restricted fan power, even if the compartment temperature Ti is not as high as would be required.

A further development of the method of operation, which avoids this disadvantage, is shown in FIG. 4. Here the switch-on step S20 is initially followed by a step S21, in which a check is made as to whether the compartment temperature Ti is smaller than the upper limit Ti⁺ of the compartment temperature to be adhered to in stationary operation. If it is, it can be assumed that the switch-on process S20 corresponds to the restoration of an operating voltage of the refrigeration device which has previously failed for a short period; In this case the method jumps to step S29. The compressor remains switched off, since no cooling down is yet necessary.

If the upper limit Ti⁺ is exceeded in step S21, the compressor is switched on in step S22. Subsequently in steps S23, S24 compartment temperature Ti and ambient temperature Ta are each compared with limit values Timax, Tamax, which, as with the method depicted in FIGS. 2 and 3, lie far above Ti⁺. If neither of the two limit values is exceeded, the duty cycle θ=1 is set in step S25; If one of the two limit values is exceeded, in step S26 a duty cycle θ<1 is defined, and the fan 5 is operated with the defined duty cycle. In step S27 a check is made as to whether the lower limit Ti⁻ of the setpoint temperature range has been reached. If not, the method returns to step S23, so that, if one of the temperatures Ti, Ta has changed, a new duty cycle θ is defined. This provides the option of also lowering the duty cycle retroactively while the appliance is starting up, if a temperature Ta increasing in the course of operation points to an overload of the compressor 3.

If it is established in step S27 that Ti has dropped below the lower limit Ti⁻ the refrigeration device has successfully started up, and in the known way in step S28 the compressor is switched off. The refrigerator now goes into stationary operation, in which only in steps S29, S31 is the compartment temperature Ti compared to the upper and lower limits Ti⁺, Ti⁻ and depending on the result of the comparison, the compressor is switched off (S28) or the compressor is switched on and the fan 5 is operated with duty cycle 1 (S30). 

1-8. (canceled)
 9. A refrigerator having a compressor, a condenser and an evaporator connected in a refrigerant circuit, with the evaporator being forced-ventilated by a fan with adjustable power, the refrigerator comprising a control unit configured to regulate the power of the fan in inverse proportion to a thermal load of the refrigerator.
 10. The refrigerator according to claim 9 wherein the control unit is configured to determine time elapsed since the refrigerator was switched on and to determine on the basis of the elapsed time since the refrigerator was switched on whether a high thermal load is present or a high thermal load is not present.
 11. The refrigerator according to claim 9 wherein the control unit is configured to control the fan based on the thermal load of the refrigerator only during a first period of operation of the compressor after the switching on the refrigeration device.
 12. The refrigeration device according to claim 9 wherein the control unit is configured to determine whether a high thermal load is present or a high thermal load is not present based on the temperature of a storage compartment.
 13. The refrigeration device according to claim 9 and further comprising an outside temperature sensor and wherein the control unit is configured determine whether a high thermal load is present or a high thermal load is not present based on the temperature detected by the outside temperature sensor.
 14. The refrigerator according to claim 9 wherein the control unit is configured to estimate a temperature of the compressor and to determine whether a high thermal load is present or a high thermal load is not present based on the estimated compressor temperature.
 15. The refrigeration device according to claim 9 wherein the compressor is operated intermittently and the regulated power is at least one of a power averaged over an operating phase and a part of an operating phase of the compressor.
 16. The refrigerator according to claim 9 wherein the control unit is configured to regulate the power of the fan by on and off sampling of the fan when the compressor is running. 